It is well known in the art to use intervertebral implants in a posterior lumbar fusion of two adjoining bodies of the vertebra. WO 95/08306 to Beckers describes such an implant for an intervertebral space. The Beckers implant comprises a body with a flat lens-shaped profile having convex surfaces. The convex surfaces are positioned on the superior and inferior, respectively, surfaces of the adjoining bodies of the vertebra, so that the greatest part of the profile of the implant coincides with the biconcave shape of the sagittal interface of the intervertebral space. In the other two planes, the body has parallel flat sides. Furthermore, an opening passes through the body parallel to its central axis, i.e. from one contact surface to the other one, so that the body can be filled with bony substance. The roundings (round edges) at the front of the body, as well as the convex contact surfaces, do not require any mechanical machining, for example milling or chiseling of the superior and inferior, respectively, surfaces of the adjoining bodies of the vertebra. The cross-sectional surface, that is perpendicular to the longitudinal axis, has two roundings, which are executed on diagonally situated corners, so that the implant can be introduced into the intervertebral space transversely, i.e., with its contact surfaces transverse to the longitudinal axis of the vertebra. Following insertion, the implant is rotated 90° with a suitable tool until the contact surfaces of the body come into contact with the superior and inferior, respectively, surfaces of the adjoining bodies of the vertebra. A disadvantage of the Beckers implant is that the contact surfaces can have a structure, with ribs extending either parallel to the longitudinal axis or transverse to it. By virtue of this structure, with teeth having symmetrical flanks, either the introduction of the implant into the intervertebral space as well as its slipping out is equally facilitated or made difficult, and the rotation of the body is equally prevented or made difficult in both directions.
Another known intervertebral implant is disclosed in U.S. Pat. No. 4,834,757 to Brantigan. The Brantigan intervertebral implant comprises a frame-like body, that has an asymmetric structure on the contact surfaces as well as on both lateral surfaces, whereby this structure comprises saw-tooth like teeth, the flanks of which are directed against the front end of the body and consequently push both adjoining bodies of the vertebra apart during the introduction of the implant into the intervertebral space. Whereas, the steep flanks hook in and thus prevent a slipping out of the implant. A disadvantage of this implant is that the teeth make rotation of the implant difficult in both directions.